Electrophotographic machines generally contain an electrophotographic photoconductor comprising a two layer coating of material on a metal substrate or photoreceptor drum. The drum itself may be anodized or may be coated with a sub-layer to assist the adhesion or binding of the two layer coating to the drum.
The two layer coating on the photoreceptor drum is made of a charge generation layer (CGL) and a charge transport layer (CTL). The CGL contains a pigment such as squaraines, phthalocyanines, azo compounds and the like dispersed in a polymeric binder. The CGL provides charge carriers or electron hole pairs upon exposure of the photoreceptor drum to light.
The CTL contains a charge transport material (CTM) selected from arylamines, hydrazones and the like and a polymeric binder material which is coated from a suitable solvent or a mixture of solvents onto the CGL. In some cases, the durability of the two layer coating is improved with a protective overcoat. Because the CTM often does not possess adequate mechanical properties, the polymeric binder is required to impart suitable mechanical properties, such as hardness, abrasion resistance and durability to the CTL. The polymeric binder for the CGL and CTL may be selected from polycarbonate, polyester, polystyrene, polyvinylchloride, polyvinyl acetate, vinyl chloride/vinyl acetate copolymers, polyvinyl acetal, alkyd resin, acrylic resin polyacrylonitrile, polyamide, polyketone, polyacrylamide, butyral resin and the like.
The use of polycarbonate-A (PCA) as a binder for the CTM has been well documented in the literature. PCA is a commercially available engineering thermoplastic, used in a variety of applications. The polymer is inert and affords good mechanical properties to the photoconductor. However, during the life of the photoreceptor drum and photoconductive coating thereon, the polymer exhibits wear under the end-seals in the cartridge containing the drum. This results in the charge-roll making contact with a ground plane of the metal of the photoreceptor drum resulting in arcing. Arcing causes severe print-defects and can shorten the life of the photoreceptor drum and coating.
Polycarbonate-Z (PCZ) has been used as an alternate binder to PCA and PCZ tends to mitigate wear problems associated with the end-seals of the photoreceptor drum. However, the use of PCZ results in wear in the paper area or circumferential surface of the photoreceptor drum in contact with paper thereby causing print-defects. Another problem observed with the use of PCZ is the presence of a high amount of residual solvent in the cured layers on the photoreceptor drum. The residual solvent slowly escapes from the CTL as the drum is used thereby causing the photoconductor coating to fatigue and the residual voltage to increase with drum life which, in turn, disadvantageously decreases the isopel optical density, i.e., the printed copies appear lighter towards the end of the photoreceptor drum life.
Blends of polycarbonates have also been suggested as binders to improve the wear properties of photoconductor coatings. For example, U.S. Pat. No. 4,851,314 to Yoshihara describes use of a mixture of polycorbonates, a high molecular weight polymer having a number average molecular weight (Mn) of 45000 or more and low molecular weight polymer having a Mn of 15000 or less with the low molecular weight polymer being present in an amount ranging from 30 to 95 parts by weight of the composition comprising the high and low molecular weight polymers.
U.S. Pat. No. 5,382,489 to Ojima et al. describes use of a mixture of polycarbonates derived from 4,4'-isopropylidenediphenol (Resin I) having a viscosity-average molecular weight ranging from 30,000 to 90,000 and a polycarbonate derived from 4,4'-cyclohexylidenebisphenol, 4,4'-(1,4-phenylenediisopropylidene)bisphenol or a copolycarbonate based on a diphenylether (Resin II) having a viscosity-average molecular weight ranging from about 20,000 to about 50,000. It is said to be necessary that Resin I have a viscosity molecular weight at least 10,000 to 20,000 higher than that of Resin II.
Despite the resins and combinations of resins suggested for use as binders in for charge transfer layers of electrophotographic machines, there remains a need for improved compositions which dramatically increase the life of the photorecepter drums without adversely affecting the quality of printed copy produced.
It is therefore an object of this invention to provide an improved compositions for use in electrophotographic applications.
Another object of the invention is to provide improved binder compositions for a photoreceptor drum of an electrophotographic machine.
A further object of the invention is to provide a charge transport layer having improved wear properties.
Another object of the invention is to provide a charge transport layer which has a higher tolerance for abrasion both in the paper area and in the seal area of the photoreceptor drum.
Still another object of the invention is to provide a composition which improves the wear properties adjacent the end seals of a photoreceptor drum and which exhibits a relatively longer coating life.
Another object of the invention is to provide an improved photoconductor coating for an electrophotographic machine which exhibits improved wear properties and life without having to increase the coating thickness of the coatings on a photoreceptor drum.